1. Field of the Invention
The present invention relates to a composition for a high refractive index plastic lens having excellent heat resistance, low water absorption and excellent surface hardness and used in various optical lenses such as eyeglass lens and the like. The present invention also relates to a lens comprising a resin obtained by polymerizing said composition.
2. Prior Art
Plastic lenses are lightweight, fragmentation-resistant and dyeable as compared with inorganic lenses, so that they have gained rapid acceptances in recent years as optical elements such as eyeglass lenses and camera lenses. Resins widely employed these days for such purposes include a resin obtained by the cast polymerization of diethylene glycol bis(allyl carbonate) (hereinafter called "DAC"). This DAC resin has various advantages such as lightweightness, superb dyeability and good workability in cutting, grinding, etc., and can respond to fashion requirements. A lens made of the DAC resin, however, has a smaller refractive index than inorganic lenses. In order for the DAC lens to have optical properties similar to those of glass lenses, it is necessary for the DAC lens to have increased central thickness, peripheral thickness and curvature, unavoidably resulting in large overall thickness. Therefore there has been a desire for the development of a resin for a lens, having a higher refractive index.
Polyurethane lenses are known as a resin lens having a higher refractive index than the DAC lens. The present inventors proposed a polyurethane lens made of a polymer between a xylylene diisocyanate compound and a polythiol compound in U.S. Pat. No. 4,775,733. This lens is in wide acceptance as optical lenses such as eyeglass lens. The present inventors also proposed a polyurethane lens of even higher refractive index in U.S. Pat. No. 5,191,055. These polyurethane lenses, however generally have poor heat resistances as compared with lenses made of a resin obtained by radical polymerization of olefin group, e.g., a DAC resin and tend to cause lens deformation in the post-treatments of lens, for example, the dyeing and surface coating of lens wherein a heat treatment is conducted generally at 60.degree.-90.degree. C. In these polyurethane lenses, therefore, the heat treatment temperature must be kept low.
With respect to the method to improve the heat resistance of polyurethane resins, there are known Japanese Patent Laid-open No. 275901/1990 and EP 408459. The polyurethane resin described in Japanese Patent Laid-open No. 275901/1990, however, has a refractive index in the range of about 1.57-1.61, which is low and insufficient. The polyurethane resin obtained by the method of Japanese Patent Laid-open No. 275901/1990 shows high water absorption and, when made into a lens of small central thickness, leads to deformation caused by water absorption at the lens center in some cases. Further, the resin has a low surface hardness and, when molded into a lens, tends to have scars. Also, since the heat resistance of the polyurethane resin described in EP 408459 is higher, it is difficult to dye it by an ordinary method when made into a lens. It is further disclosed in EP 408459 that when there is used, as the thiol component, pentaerythritol which is the same as tetrakis(mercapto-methyl)methane used in the present invention, use of xylylene diisocyanate as the isocyanate component is not preferable.
Further in these prior techniques, the thiol component used for heat resistance improvement becomes a solid in some cases. For example, tetrakis(mercapto-methyl)methane (this is preferably used in the present invention) is a solid having a melting point of 72.degree.-73.degree. C. and, when used singly as the thiol component as in EP 408459, makes the handling very complicated in the molding of lens. Also, the compound represented by general formula (3): ##STR3## described in Japanese Patent Laid-open No. 275901/1990 has a high viscosity ordinarily and therefore, when tetrakis(mercaptomethyl)methane as the component (c) is added in an increased proportion, crystals appear, making the handling complicated and reducing the workability. Moreover, the restriction in proportion of tetrakis-(mercaptomethyl)methane added makes impossible the sufficient improvement in heat resistance.